Microelectronic circuitry, in the form of integrated circuits have utilized isolation trenches filled with electrically isolating materials to electrically isolate separate integrated circuit structures from one another. These same isolation trench structures may be doped to provide current paths between circuit structures. Furthermore, in doped wafers, the walls and floors of isolation trenches are heavily doped to increase their isolating effect. Abrupt transitions between active and isolation regions are necessary to maximize circuit density. Shallow trench isolation is one means for achieving the necessary abrupt transition. Existing technologies for trench fabrication stress the need for creating trenches with nearly vertical trench walls. The need for steep walled narrow trenches presents difficulties in doping the trench walls and bottoms. Thus, the need for doping and the need for deep narrow trenches are at cross purposes. Additionally, these nearly vertical walls create sharp angles (nearly 90.degree.) at the trench corners (see angle .phi. of FIG. 1) which cause enhanced edge conduction. These sharp angles are known to cause electrical failures in integrated circuits and are a cause of circuit unreliability. This problem is known as the "edge effect".
Typical trench fabrication methods feature the etching of vertical walled trenches between active circuit areas, then filling the trenches with electrically isolating material (typically silicon dioxide), thereby electrically isolating independent circuit portions.
Other isolation processes exhibit similar problems along with unique problems of their own. For example, the so called LOCOS method, which selectively oxidizes patterns on a silicon substrate to create isolation regions, suffers from oxide encroachment of silicon dioxide under the silicon nitride layer. This oxide encroachment is known in the industry as a bird's beak. This bird's beak problem reduces the active circuit area between the isolation regions by encroaching into the active regions. As feature size decreases this becomes a more significant problem. Other known processes include a variation of the LOCOS process which is known in the industry as SWAMI, as developed by Hewlett Packard. The SWAMI process includes outwardly etching the silicon nitride and the pad oxides at 60 degrees such that the subsequent oxidation and beak formation length is limited. Other methods include using high-pressure oxidation techniques where the oxidation is grown faster and at a lower temperature which results in minimizing the growth of the bird's beak. But the end result is, that all of these methods suffer from the bird's beak problem which serves to restrict the active surface area putting a limit on circuit density.
The primary object of the present invention is to provide a sloped wall isolation trench which is more easily doped, thereby creating a more effective isolation trench. Another important object is the fabrication of sloped wall isolation trenches which reduce the edge effect and thereby increase circuit reliability.
A related object of the present invention is to provide fabrication processes for producing microelectronic integrated circuit structures featuring sloped wall isolation trenches in accordance with the forgoing objects.
Another related object of the present invention is to provide a sloped wall isolation trench which does not suffer from the bird's beak problem thereby conserving the maximum amount of microelectronic integrated circuit surface area.